chore(flow-client): remove the GWT client engine and build wiring

[totally-not-ai]

Summary

With the TypeScript engine now live (previous PR), the GWT client is dead. Removes it: com.vaadin.client.* sources + their JUnit/GWTTestCase tests, the ClientEngine*.gwt.xml modules, ClientEngineLinker + its single-script template, and the GWT deps / gwt-maven-plugin / GWT test wiring from pom.xml and package.json.

flow-client is now a TypeScript + resources module with no Java sources. Nothing outside flow-client referenced com.vaadin.client. Large but mechanical (deletions).

Stacked on #24953 (base flow-client-ts-cutover) — merge after it. Non-draft; please review before merging.

[totally-not-ai]

Review — GWT client engine + build wiring removal

The removal is complete and internally consistent:

• All GWT Java is gone — after this PR flow-client/src/main/java contains 0 .java files, and src/test-gwt/ is removed entirely (28 files). The 30 deleted src/test files are the GWT-only unit tests.
• pom.xml cleanly strips every piece of GWT wiring: the gwt.module* properties, the com.google.gwt dependency import + gwt-user/gwt-dev/gwt-elemental, the gwt-maven-plugin (compile/generateAsync/test), the build-helper src/test-gwt/java source dir, the surefire **/Gwt*.java exclude, the sdm profile, and the slow-tests gwt.test.pattern. The still-needed FlowClient.js spotless exclude is correctly retained.
• No dangling references — a repo-wide sweep finds 0 remaining import com.vaadin.client.* in any Java file, so removing the engine breaks no remaining compilation.

One follow-up that this removal breaks (please address or confirm handled elsewhere)

flow-tests/test-root-context/.../dependencies/DependenciesLoadingAnnotationsIT.java uses the GWT client engine script as its boundary marker between Flow and user dependencies:

if (jsUrl.endsWith(".cache.js") && jsUrl.contains("static/client/client-")) {
    foundClientEngine = true;
}

With the engine gone, foundClientEngine never flips to true, so every script is treated as a "flow dependency" and the loop asserts "Flow dependencies should not contain user dependencies" against the first eager/lazy user script — which will now fail (or, at best, silently stop testing the ordering it was written to verify). This IT isn't touched by this PR but is a direct consequence of the removal; it needs updating (new marker for the TS FlowClient, or a rewrite of the ordering check).

Minor cleanup leftovers (non-blocking)

• flow-test-generic/.../ClassesSerializableTest.java still lists com.vaadin.flow.linker.ClientEngineLinker (+$Script) in its exclusion regex list — the class no longer exists, so the pattern is now dead (harmless string, but could be removed).
• ApplicationConstants.CLIENT_ENGINE_PATH is now unused by production code (see the 24955 note); safe to keep as public shared API, but worth noting.

Otherwise LGTM — thorough, correct removal with no dangling imports or build references.